Abstract
Coastal ecosystem modifications have contributed to the spread of introduced species through alterations of historic disturbance regimes and resource availability, and increased propagule pressure. Frequency of occurrence of the Manila clam (Venerupis phillipinarum, Veneridae) in Southern California estuaries has increased from absent or sparse to common since the mid-1990s. Potential invasion vectors include seafood sales and aquaculture, and spread from established northern populations over decades. The clam’s post-settlement habitat preferences are, however, uncertain in this region. Our project aimed to identify factors associated with established patches of the clam within a bay toward the southern end of this introduced range. During summer 2013, we sampled 10 tidal flat sites in Mission Bay, San Diego; each containing an area with and without hard structure (e.g., riprap, boulders). We measured likely environmental influences (e.g., sediment variables, distance to ocean). Manila clam densities across the bay were most strongly associated with site, where highest densities were located in the northern and/or back halves of the bay; and weakly correlated with lower porewater salinities. Within sites, Manila clam density was enhanced in the presence of hard structure in most sites. Prevailing currents and salinity regimes likely contribute to bay wide distributions, while hard structures may provide suitable microhabitats (refuge from predators and physical stress) and larval entrapment within sites. Results provide insights into decisions about future shoreline management efforts. Finally, we identify directions for future study to better understand and therefore predict patterns of establishment of the Manila clam in the southern portion of its introduced range.
Highlights
Humans play key roles in the spread and subsequent establishment of species from native to introduced ranges around the world [1]
While establishment and spread tend to be heaviest from San Francisco Bay northward [2],[10], the Manila clam appears to be increasing in Southern California based on comparisons among recent surveys, in which the clam was found to be relatively common (e.g., [11],[12]), and earlier studies conducted in the same areas that did not find the clam [2], [13],[14]
Organic matter content and porewater salinity were similar between areas with and without hard structure, and only differed with site, but with no strong estuarine gradients from mouth to back of the bay (Table 1A, Fig. 2A)
Summary
Humans play key roles in the spread and subsequent establishment of species from native to introduced ranges around the world [1]. Expansion throughout the northwest coast, from British Columbia to Northern California, has been due to planktonic larval dispersal, accidental introductions with transplanted oysters, and intentional introductions of transplanted clams for aquaculture [5,6,7,8,9]. While establishment and spread tend to be heaviest from San Francisco Bay northward [2],[10], the Manila clam appears to be increasing in Southern California based on comparisons among recent surveys, in which the clam was found to be relatively common (e.g., [11],[12]), and earlier studies conducted in the same areas that did not find the clam [2], [13],[14]. Vectors of spread in the southern end of this range probably include sale of live clams, aquaculture [15], and transport from the northern established populations. Influences on establishment within Southern California bays, remain uncertain
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